v1.2.0 to fix multiple-definitions linker error

### Releases v1.2.0

1. Fix `multiple-definitions` linker error. Drop `src_cpp` and `src_h` directories
2. Add example [multiFileProject](examples/multiFileProject) to demo for multiple-file project.
3. Optimize library code by using `reference-passing` instead of `value-passing`
4. Update all examples

examples/multiFileProject/multiFileProject.ino

@@ -0,0 +1,38 @@
+/****************************************************************************************************************************
+  multiFileProject.ino
+  For Teensy boards
+  Written by Khoi Hoang
+
+  Built by Khoi Hoang https://github.com/khoih-prog/Teensy_TimerInterrupt
+  Licensed under MIT license
+
+  Now even you use all these new 16 ISR-based timers,with their maximum interval practically unlimited (limited only by
+  unsigned long miliseconds), you just consume only one NRF52 timer and avoid conflicting with other cores' tasks.
+  The accuracy is nearly perfect compared to software timers. The most important feature is they're ISR-based timers
+  Therefore, their executions are not blocked by bad-behaving functions / tasks.
+  This important feature is absolutely necessary for mission-critical tasks.
+*****************************************************************************************************************************/
+
+// To demo how to include files in multi-file Projects
+
+#if !( defined(ARDUINO_ARCH_RP2040) || defined(ARDUINO_RASPBERRY_PI_PICO) || defined(ARDUINO_ADAFRUIT_FEATHER_RP2040) || defined(ARDUINO_GENERIC_RP2040) ) && !defined(ARDUINO_ARCH_MBED) 
+  #error This code is intended to run on the non-mbed RP2040 arduino-pico platform! Please check your Tools->Board setting.
+#endif
+
+#include "multiFileProject.h"
+
+// Can be included as many times as necessary, without `Multiple Definitions` Linker Error
+#include "RPi_Pico_TimerInterrupt.h"
+
+// To be included only in main(), .ino with setup() to avoid `Multiple Definitions` Linker Error
+#include "RPi_Pico_ISR_Timer.h"
+
+void setup() 
+{
+  // put your setup code here, to run once:
+}
+
+void loop() 
+{
+  // put your main code here, to run repeatedly:
+}